When making a telephone call in a noisy environment the audibility of the far-end speech may at times be low, since the loudspeaker signal may be drowned by the background noise at the near-end. At such occasions, it would be beneficial to increase the loudness of the far-end speech signal in order to improve the audibility.
Most mobile terminals used today include a manual volume control that can be used for increasing the loudspeaker volume. However, this may not always solve the problem of enhancing the audibility, and that mainly depends on the so-called side-tone that most often is included in a mobile terminal. The side-tone adds a portion of the talkers voice to the loudspeaker in order to make the terminal sound “more alive”. Typically, the side-tone is at a level of approximately 10–20 dB below the microphone level, see ITU-T Recommendation G.12I. The perception of the side-tone is very sensitive to the time-delay between the side-tone and the talkers voice. This time-delay should preferably be short, hence the side-tone has traditionally been implemented as an attenuated (by 10–20 dB) version of the microphone signal.
Although the side-tone is intended to add the talkers voice to the loudspeaker, any background environmental noise may also be added to the loudspeaker via the side-tone, thus causing the far-end voice to be contaminated by near-end noise. If the near-end background environmental noise is very high, this may cause the audibility of the far-end speech to be reduced due to a low signal-to-noise ratio. In this situation it is desirable for the user to increase the level of the far-end speech. Depending on the implementation of the volume control in the terminal, this may or may not increase the audibility. If the volume control is implemented (digitally) on the down-link signal and the side-tone level is not connected to the volume level, then the signal-to-noise ratio of the far-end speaker will be increased by this action. Should, however, the volume control be implemented as an analogue amplification of the loudspeaker signal, or the side-tone level is dependent on the volume setting of the phone, then the mobile terminal user is not helped by any manual volume control in the handset, since this will amplify both the far-end speech and the near-end noise stemming from the side-tone, and the signal-to-noise ratio will remain at a constant value regardless of the loudspeaker volume level.
The problem of increasing the audibility has been addressed in U.S. Pat. No. 5,5524,148, Allen et al. by including a processing function in the network that amplifies the far-end signal based on the noise level present at the mobile user side. Since this amplification only affects the far-end signal and not the noise signal added locally in the terminal via the side-tone, this approach will increase the signal to noise ratio.
The solution described in U.S. Pat. No. 5,5524,148, Allen et al. amplifies the far-end signal based on the noise level from the near-end. This solution is based on the assumption that whenever a user is present in an environment with a high background noise level, the user will benefit from an increased level of the far-end signal.
This assumption may be valid if the user is using a mobile terminal with a traditional design, i.e. a mobile handset where the side-tone is implemented as an attenuated replica of the microphone signal. However, with the increased diversity in the design of mobile terminals, there may be several situations when the user does not have any problem with the audibility of the far-end speaker, although the user is positioned in an environment with a high background noise. Examples of terminal equipment where there possibly are no problems with the audibility although used in a noise environment are:    1. Handsets with volume control that does not affect the side-tone level or with more advanced side-tone implementations (including noise suppressing techniques in the side-tone).    2. Hands-free accessories designed for noisy conditions (such as car hands-free kits with loudspeaker volume and frequency characteristics dependent on the environmental noise) and for which there are no side-tones added to the loudspeaker.    3. Headsets or ear-plugs which reduce the perceived environmental noise level.    4. Handsets with active noise control.
This should not be seen as an exclusive list, but serve as examples of terminal equipment that are designed to operate properly in environments with a high acoustical background noise.
In situations where the terminal equipment has been designed to operate in noisy conditions, any level adjustment made in the network may degrade the overall perceived quality of the conversation. Firstly, raising the level of the far-end speech may cause the terminal equipment at the near-end to not function as designed. Secondly, and perhaps most important, the speech and talker levels in a conversation is coupled via the human perception of speech levels. As outlined in the following, any amplification of the signal in the network may influence the speech levels in a non-desired way if implemented without specific knowledge of the situation.
In a conversation, a talker tends to adjust the level of the voice to the level of the other party and also to the background noise. Similarly, when exposed to speech in a telephone conversation via a loudspeaker (either in a handset, hands-free or headset), a talker tends to adjust the voice level to a similar level as perceived from the loudspeaker. Thus, if the level of the far-end speech is raised the near-end user is also likely to unconsciously increase the voice level. If the terminal equipment at the near-end has been designed to work properly even in an environment with a high background noise, this increase of the voice level is affecting the user in a negative way (e.g. more strain to the voice) without adding any benefit of increased audibility.
An example of the problem described in the previous section is when using a headset in a noisy environment. With a proper design of the headset, the user is likely to have no problems hearing the far-end talker at nominal signal levels and is likely to talk at a comfortable and natural speech level. Should, however, the level of the far-end talker be increased by any network equipment (due to the measurement of a high background noise level from the near-end), the near-end talker may unconsciously also increase the voice level with more strain to the voice and less privacy as an effect.
In the example above it should be noted that with a headset the user is less exposed to the background noise and the voice is not raised due to the high level of the background noise, as would be the case if using an ordinary handset. When using an ordinary handset in a noisy environment and being fully exposed to the background noise and having a side-tone in the handset with a high noise level, the near-end user would benefit from having an increased far-end speech level, as outlined in the previous section.
Thus, there is a need for an improved method for enhancing the audibility for users of ordinary mobile handset without degrading the overall performance in situations with terminal equipment designed for environments with high near-end background noise.